1. Field of the Invention
This invention relates to an anti-skid control device which, in a wheel braking operation, controls wheel braking forces so as to prevent the locking of the wheels.
2. Description of the Related Art
In general, with a recent anti-skid control device, the brake fluid pressures in four wheel brakes--a front right wheel brake, a front left wheel brake, a rear right wheel brake, and a rear left wheel brake--are decreased or increased independently of one another, and three brake control systems, namely, a front left wheel brake control system, a front right wheel brake control system, and a rear right and left wheel brake control system are established; or four brake control systems, namely, a front left wheel brake control system, a front right wheel brake control system, a rear left wheel brake control system, and a rear right wheel brake control system are established.
A variety of control devices have been proposed in the art which control (increases and decreases) the brake fluid pressures in four wheel brakes independently of one another. Among those control systems, the system disclosed by Japanese Patent Unexamined Publication No. Hei 6-171487 is small in the number of components and low in manufacturing cost.
The control device is designed as follows:
In a first main path which hydraulically connects the front left wheel brake and the rear right wheel brake to a first pressure chamber which is one of the two pressure chambers in a master cylinder, there are arranged a normally open first electromagnetic valve which selectively isolates only the front left wheel brake from the first pressure chamber, and a normally open second electromagnetic valve which selectively isolates only the rear right wheel brake from the first pressure chamber;
in a second main path which hydraulically connects the front right wheel brake and the rear left wheel brake to a second pressure chamber which is the other of the two pressure chamber in the master cylinder, there are arranged a normally open third electromagnetic valve which selectively isolates only the front right wheel brake from the second pressure chamber, and a normally open fourth electromagnetic valve which selectively isolates only the rear left wheel brake from the second pressure chamber;
a first circulating path is provided which bypasses the first electromagnetic valve, to circulate the brake fluid in the front left wheel brake to the first main path between the first pressure chamber and the first and second electromagnetic valves, and which bypasses the second electromagnetic valve, to circulate the brake fluid in the rear right wheel brake to the first main path between the first pressure chamber and the first and second electromagnetic valves;
a second circulating path is provided which bypasses the third electro-magnetic valve, to circulate the brake fluid in the front right wheel brake to the second main path between the second pressure chamber and the third and fourth electromagnetic valves, and which bypasses the fourth electromagnetic valve, to circulate the brake fluid in the rear left wheel brake to the second path between the second pressure chamber and the third and fourth electromagnetic valves;
in the first circulating path, a first hydraulic pump is provided which sends brake fluid from the front left wheel brake and the rear right wheel brake to the first main path between the first pressure chamber and the first and second electromagnetic valves;
in the second circulating path, a second hydraulic pump is provided which sends brake fluid from the front right wheel brake and the rear left wheel brake to the second main path between the second pressure chamber and the third and fourth electromagnetic valves;
in the first circulating path between the front left wheel brake and the first hydraulic pressure pump, a first orifice is provided which increases and decreases the brake fluid pressure in the front left wheel brake in response to the activation and deactivation of the first electromagnetic valve with the master cylinder and the first hydraulic pressure pump in operation;
in the first circulating path between the rear right wheel brake and the first hydraulic pressure pump, a second orifice is provided which increases and decreases the brake fluid pressure in the rear right wheel brake in response to the activation and deactivation of the second electromagnetic valve with the master cylinder and the first hydraulic pressure pump in operation;
in the second circulating path between the front right wheel brakes and the second hydraulic pressure pump, a third orifice is provided which increases and decreases the brake fluid pressure in the front right wheel brake in response to the activation and deactivation of the third electromagnetic valve with the master cylinder and the second hydraulic pressure pump in operation; and
in the second circulating path between the rear left wheel brake and the second hydraulic pressure pump, a fourth orifice is provided which increases and decreases the brake fluid pressure in the rear left wheel brake in response to the activation and deactivation of the fourth electromagnetic valve with the master cylinder and the second hydraulic pressure pump in operation.
However, the above-described control device including three or four brake control systems suffers from the following problem: With the control device, one hydraulic pressure pump decreases the brake fluid pressures in the front and rear wheel brakes. Hence, when the brake fluid pressure in the rear wheel brake which consumes a relatively small quantity of brake fluid is increased; that is, when it is high, the pressure decreasing capacity of the front wheel brake which consumes a relatively large quantity of brake fluid is lowered. As a result, the restoration in speed of the front wheel is delayed, so that the front wheel is liable to be locked.